where α = 95570, β = -0.64 are extracted by fitting the curve presented in Figure 4c. The range of resistance change under pressure is between ~1 kΩ and ~20 kΩ, which meets the series resistance required by the VO2 oscillation neuron. This piezoresistive sensor can thus be combined with the VO2 oscillation neuron to emulate artificial haptic perception, where the sensor is used as a receiver of pressure signals and the resultant sensory signal is converted into spike trains by the VO2 neuron.
          Indeed, when different pressures/weights (100, 150, 200, 250 g) are applied to the sensor with a constant bias voltage (5 V, 20 μs), the oscillation neuron exhibits different output spike frequencies (0.45, 0.55, 0.75, 0.95 MHz), as shown in Figure 4d. The converted spike frequency increases as the pressure increases. In this way, the artificial haptic sensory neuron can directly respond to pressure signals and encode them into spikes, and the output spikes can then transmit information to spiking neural networks for further processing.
          Haptic perception allows human to recognize objects, discriminate texture, and react appropriately in a social exchange.[2, 40] This essentially requires integration of multiple spatial correlated sensory stimuli. In order to achieve this, we combine two sensors in parallel as a proof of concept, and they are further connected in series with a VO2 memristor (the circuit structure is shown in Figure S10, Supporting Information). This is utilized to recognize Braille characters in the present study. As shown in Figure 4e, the black circles represent convex patterns in the Braille characters, while the white circles indicate no convex, which correspond to the cases of sensing pressure and no pressure when being touched, respectively. These scenarios were emulated in experiment by applying 100 g for convex patterns and 0 g otherwise. The results in Figure 4e show that when only one of the two sensors are triggered, the output oscillation frequency of VO2 neuron is ~0.4 MHz. However, when the two sensors are triggered at the same time, the output oscillation frequency will be higher (~1.1 MHz). The output frequency is zero when neither of the sensors is triggered (detailed information is shown in Supplementary Video 1-3). Therefore, the Braille characters can be read out from the different patterns of output frequencies produced by the VO2 neurons. It should be pointed out that some Braille characters may not be fully distinguished based on the horizontal inputs, and in this case an additional process can be introduced to apply vertical inputs onto the device so as to further distinguish them.